1. Field of the Invention
The present invention relates to an electrode plate for a plasma etching and a plasma etching apparatus, and more particularly, to an arrangement of gas holes in the electrode plate.
2. Description of the Related Art
As an upper electrode of a plasma etching apparatus, an electrode plate (CEL) on a side of a plasma generating space and a cooling plate on an opposite side to the plasma generating space are provided to be adjacent to each other. The electrode plate is a disc-shaped member formed of silicon (Si) to a predetermined thickness, and a plurality of gas holes penetrate through the electrode plate in order to introduce gas into the plasma generating space. Each of the gas holes is a pore, and the gas holes are provided on circumferences of a plurality of concentric circles at regular pitches. Due to the above structure, the electrode plate may function as a shower head for introducing an etching gas into the plasma generating space.
The silicon (Si) forming the electrode plate is worn away by the plasma. Generally, when the electrode plate is used for hundreds of hours, inner portions of the gas holes are worn away by the plasma, and thus, the gas holes are widened toward the plasma generating space similar to trumpet shapes. When the gas holes are widened, etching rates at a center portion and at an edge portion of the electrode plate become uneven, and thus, process characteristics are changed. In addition, the plasma is likely to be introduced into the gas holes. Ions or electrons in the plasma introduced into the gas holes proceed against a flow of the gas in the gas holes, and excite gas in a gap between the electrode plate and the cooling plate. Accordingly, abnormal discharge generates between the electrode plate and the cooling plate, or at a ceiling surface of a process chamber. The abnormal discharge may damage the electrode plate or the cooling plate, may become a source of particles, or may damage an Alumite film that covers a surface of the electrode plate or the cooling plate, and accordingly, an interior of a chamber is contaminated.
For example, Patent Reference 1 discloses a technology of providing magnets that generate a point cusp magnetic field on a surface side of the electrode plate opposite to the other surface of the electrode plate facing a substrate held by a holding stage, and positioning the gas holes so that locations of the gas holes and the magnets in the electrode plate are not in line with each other and thus, the gas holes may be provided at locations where the magnetic field becomes zero, to address a problem that diameters of the gas holes are increased due to the plasma. According to this structure, the gas holes may be provided in portions where a plasma density is low, and thus, the gas holes may not be exposed to highly concentrated plasma. Accordingly, generation of an abnormal discharge on a rear surface of the upper electrode may be prevented.
In Patent Reference 1, four magnets including two N-poles and two S-poles are disposed so that the N-poles and the S-poles generating a point cusp magnetic field are alternately disposed, and the gas holes are provided in a zero-magnetic field portion that is generated at a center of the four magnets. However, the gas holes may not be stably provided at the portion of zero-magnetic field. Therefore, according to Patent Reference 1, the abnormal discharge generating on the rear surface of the upper electrode may not be completely prevented.
3. Prior Art Reference    (Patent Reference 1) Japanese Patent Laid-open Publication No. 2003-31555